Digital communication receivers must sample an analog waveform and then reliably detect the sampled data. Signals arriving at a receiver are typically corrupted by intersymbol interference (ISI), crosstalk, echo, and other noise. Thus, receivers must jointly equalize the channel, to compensate for such distortions, and decode the encoded signals at increasingly high clock rates. Decision-feedback equalization (DFE) is a widely-used technique for removing intersymbol interference and other noise. For a detailed discussion of decision feedback equalizers, see, for example, R. Gitlin et al., Digital Communication Principles, (Plenum Press 1992) and E. A. Lee and D. G. Messerschmitt, Digital Communications, (Kluwer Academic Press, 1988), each incorporated by reference herein. Generally, decision-feedback equalization utilizes a nonlinear equalizer to equalize the channel using a feedback loop based on previously decided symbols.
In one typical DFE implementation, a received analog signal is sampled and compared to one or more thresholds to generate the detected data. A DFE correction, v(t), is subtracted in a feedback fashion to produce a DFE corrected signal w(t). Historically, the same clock, generated from the received signal by a clock and data recovery (CDR) circuit, was used to sample the incoming signal and for the DFE operation. It has been recognized, however, that the data sampling and DFE operations may have different optimal sampling points. Thus, a number of techniques have been proposed or suggested for sampling the DFE latches with a sampling phase that may be offset from the normal CDR data clock sampling phase. For example, it has been suggested to manually establish the DFE clock offset. The optimal DFE sampling phase, however, is dependent on the channel or other equalizer settings. Thus, optimizing the sampling phase across a range of programmable values becomes a burden.
A need therefore exists for methods and apparatus for adaptively establishing the optimal sampling phase offset for the DFE operation.